The invention relates to optical communication networks and particularly to networks wherein channels are added to and dropped from the network at different locations or nodes.
In traditional point-to-point WDM optical communication networks, wherein each wavelength represents a channel all channels are multiplexed together at a sending node, transmitted through the network and then demultiplexed at the receiving node. The multiplexer and demultiplexer typically have substantially the same loss for all wavelengths, so that performance will not vary significantly between channels. However, in systems wherein individual channels are added and dropped at intermediate nodes within a network, the link losses for each channel, that is the power losses imposed on a channel between a transmitter and receiver, will differ depending on the path taken. This applies to all types of network configuration, including point-to-point and ring networks. A channel experiences losses through the length of optical fibre, at each add/drop filter both when the channel is added to or dropped from the transmission path and also when it is reflected or transmitted through a filter.
In order to reduce the losses in the different channels, filter components are selected to have low add/drop losses and also low pass-through losses. A number of different filter technologies exist which are capable of performing this function. When a few channels are dropped or added at a node, thin film filters or fibre grating based filters are commonly used. These filters are then stacked, so that all channels passing through the node will be subjected to the pass-through losses of each filter.
EP 0 905 936 describes an optical fibre network having nodes that allow the passive pass-through of channels. Losses at each node are reduced by means of add/drop filters which add and drop wavelength bands, respectively. The add and drop filters each consist of a single optical interference filter. The component channels are combined or separated using a multiplexer connected upstream of the add filter and demultiplexer connected downstream of the drop filter, respectively. While the arrangement described in this prior art reference reduces the losses within a network, some channels may nevertheless have a link loss that exceeds system requirements. For such channels a semiconductor optical preamplifier or power amplifier or both can be utilised. For networks using only passive components between the transmitter and receiver, the network length is limited by the channels with the highest link losses.
In the light of prior art arrangements it is an object of the present invention to provide an optical communications network, in which the channel losses are controlled so as to optimise the network transmission distance.
This is achieved by distributing the losses occurring within a network more evenly between multiple channels. More specifically, the invention relates to a network capable of carrying multiple WDM channel wavelengths, wherein multiple nodes are connected via a transmission medium and have filter elements for adding channel wavelengths transmitted by said node to said transmission medium and/or dropping channel wavelengths destined for said node from said transmission medium. All other wavelengths are passively relaying through the filter elements but sustain a loss of power thereby. At least two nodes in the network have two or more filter elements for either adding or dropping channel wavelengths. The filter elements within these two nodes are arranged such that those channels that have the highest link losses are relayed through as few add/drop filter elements as possible. In particular, filter elements are arranged such that received channel wavelengths with a high link loss relative to an allowed link loss are dropped from said transmission medium upstream of received channel wavelengths with a low link loss relative to an allowed link loss. Similarly, for channel wavelengths originating or transmitted by the node, the filter elements are arranged such that transmitted channel wavelengths with a high link loss relative to an allowed link loss are added to said transmission medium downstream of transmitted channel wavelengths with a low link loss relative to an allowed link loss. The link loss is the loss of power sustained throughout the transmission path of a channel and may include losses both upstream and downstream of the add/drop filter elements. The link loss is expressed as a quantity relative to an allowed link loss, and most preferably as a proportion of an allowed link loss. Thus the nodes may be configured such that those channels that require a lower link loss, for example due to higher bit rates, and have a low absolute link loss compared to other channels are nevertheless still spared additional losses due to add/drop filter elements that the channel is less able to tolerate.
By imposing the additional losses caused by the passive pass-through of the add/drop filters on channels that have losses well below the maximum level imposed by system requirements, those channels with high link losses relative to an allowed link loss and which therefore limit the network length, are capable of tolerating additional loss elsewhere, possibly as additional transmission distance.
In accordance with a further aspect of the invention, a network node is proposed having at least two combining elements, at least one of these elements being capable of adding a waveband containing multiple channel wavelengths. This band combining element includes a first filter for adding the specified waveband and a multiplexer coupled to the first filter. The multiplexer may consist of a series of multiple cascaded fine filters. The fine filters are each adapted to pass a specified channel wavelength and passively transmit, or reflect, the other wavelengths within the waveband. The combining elements are arranged such that channel wavelengths having a high link loss relative to an allowable link loss are added to the transmission line downstream of channels wavelengths having a lower link loss relative to an allowable link loss.
The invention further resides in node of a WDM network having at least two wavelength extracting elements, at least one of these elements being capable of dropping a waveband containing multiple channel wavelengths. This band extracting element includes a first filter for dropping the specified waveband and a demultiplexer coupled to the first filter. The demultiplexer preferably consists of a series of multiple cascaded fine filters. The fine filters are each adapted to pass a specified channel wavelength and passively transmit, or reflect, the other wavelengths within the waveband. The extracting elements are arranged such that channel wavelengths having a high link loss relative to an allowable link loss are dropped from the transmission line upstream of channels wavelengths having a lower link loss relative to an allowable link loss.